Endocrine: most common, classical mode, hormones delivered to target cells by blood.
Paracrine: hormone released diffuses to its target cells through immediate extracellular space. Blood is not directly involved in the delivery.
Neuroendocrine: hormone is produced and released by a neuron., delivered to target cells by blood.
Autocrine: hormone released feeds-back on the cell of origin, again without entering blood circulation.
HORMONE TARGET CELL SPECIFICITY
Only target cells, or cells that have specific receptors, will respond to the hormone’s presence. The strength of this response will depend on:
-blood levels of the hormone. – the relative numbers of receptors for that hormone on or in the target cells.
The affinity (or strength of interactions) of the hormone and the receptor.
Half-life onset and duration of hormone activity..
The affinity of hormones to their specific receptors is typically very high. The actual concentration of a circulating hormone in blood at any time reflects. – its rate of release – the speed of its inactiviation and removal from the body.
The half –life is the time required for the hormone to loose half of its original effectiveness(or drop to half of its orginal concentration)
The time required for hormone effects to take place varies greatly, from almost immediate responses to hours or even days.
In addition, some hormones are produced in an inactive form and must be activated in the target cells before exerting cellular responses.
In terms of duration of hormone action, it ranges from about 20 minutes to several hours, depending on the hormone.
CONTROL OF HORMONE RELEASE:
The synthesis and secretion of most hormones are usually regulated by negative feedback systems. As hormone levels rise, they stimulate target organ responses. These in turn, inhibit further hormone release. The stimuli that induce endocrine glands to synthesize and release hormones belong to one of the following major types.
Humoral
Neural
Hormonal
The endocrine system works closely with the nervous system to maintain and steady state of the body.the functions of hormones. Endocrin gland is ductless gland. Target issue. Chemical structure of hormones: protein, peptide, aminoacid derived and steroid hormones. Pheromones
Showing posts with label hair. Show all posts
Showing posts with label hair. Show all posts
hairs biology
The "hair" is not just that you have hair on the head. You have "hair" (and hair) in almost all parts of the body. (The places where you have no hair are the lips, the palms and soles).
Most of the hair that you have in your body is easy to see, such as the eyebrows, head, arms or legs. But the hair you have on, for example, the cheeks of the face is almost invisible. Depending on the location, hair fulfills different functions. The hair that you have in your head keeps the body heat of this and gives some protection against your skull blows. Eyelashes protect your eyes by reducing the amount of light and dust that can penetrate them; and eyebrows protect the eyes of sweat that can drip on your forehead.
Most of the hair that you have in your body is easy to see, such as the eyebrows, head, arms or legs. But the hair you have on, for example, the cheeks of the face is almost invisible. Depending on the location, hair fulfills different functions. The hair that you have in your head keeps the body heat of this and gives some protection against your skull blows. Eyelashes protect your eyes by reducing the amount of light and dust that can penetrate them; and eyebrows protect the eyes of sweat that can drip on your forehead.
SEBACEOUS GLANDS
A. Structure and Location: These exocrine glands occur in all thin skin, most often in association with hair follicles into which their ducts empty, but are most numerous in the skin of the face, forehead, and scalp. In hairless skin, they open directly onto the surface. Their acinar secretory portions contain many large lipid-filled cells that appear pale-staining and foamy.
B. Function: The acinar cells of sebaceous glands fill with lipid droplets containing a mixture of triglycerides, waxes, squalene, and cholesterol and its esters. Their nuclei become pyknotic, and the cells eventually burst, releasing their contents and other cell debris (together termed sebum) into the ducts. The entire cell is shed, a type of secretion known as bolocrine secretion. The oily sebum moves through the ducts and into the hair follicle. It covers the hair and moves out onto the surface. Here, it lubricates the skin and may have some antibacterial or antifungal effects. The secretory activity of these glands, which begin functioning at puberty, is continuous and is increased by androgens.
B. Function: The acinar cells of sebaceous glands fill with lipid droplets containing a mixture of triglycerides, waxes, squalene, and cholesterol and its esters. Their nuclei become pyknotic, and the cells eventually burst, releasing their contents and other cell debris (together termed sebum) into the ducts. The entire cell is shed, a type of secretion known as bolocrine secretion. The oily sebum moves through the ducts and into the hair follicle. It covers the hair and moves out onto the surface. Here, it lubricates the skin and may have some antibacterial or antifungal effects. The secretory activity of these glands, which begin functioning at puberty, is continuous and is increased by androgens.
Root sheaths
The concentric sheaths surrounding the hair shaft are more clearly distin- guished in the area between the bulb and the skin surface. a. Internal root sheath. The layer closest to the hair shaft, it extends only from the bulb to the level of the sebaceous gland ducts. At this point the soft keratin-filled cells are shed into the follicular canal. There are 3 component layers: the cuticle of the internal root sheath is a layer of flat cells separated from the hair shaft cuticle only by the follicular canal; the middle layer is Huxley's layer, comprising one to 3 layers of low cuboidal cells; the outermost layer is Henle's layer, a translucent layer of flattened to cuboidal cells resembling the epidermal stratum lucidum. b. External root sheath. This surrounds the internal root sheath and is continuous with the epidermis. Above the level of the sebaceous glands, it includes all the epidermal layers. Below this level, it retains only the granulosum, spinosum, and basale. The granulosum is also lost near the follicle's base, where the spinosum and basale become continuous with the layers of the germinal matrix. c. Glassy membrane. This is the thickened basal lamina underlying the stratum basale of the external root sheath and separating it from the surrounding connective tissue sheath. d. Connective tissue sheath. A layer of condensed connective tissue, this surrounds the entire follicle, including the bulb. It extends along the follicle to the surface, where it blends into the looser papillary dermis
B. Follicle and Hair Structure
Hair follicles extend from the surface deep into the dermis or hypodermis. The follicle's broad base, or hair bulb, consists of a cap of rapidly dividing epithelial cells (the germinal matrix) overlying a dermal papilla that harbors the nerve and blood supply. Cells from the germinal matrix keratinize, forming the concentric layers of the hair shaft as they move toward the surface. Near the surface, distinct layers can be seen ensheathing the canal that contains the hair shaft. Integumentary system
HAIR
nHair occurs only in thin skin; its color, size, shape, and distribution vary according to race, age, sex, and body region. The structures in skin that form hairs and maintain their growth are called hair follicles.
nA. Follicle and Hair Development:
1. Follicles. Early in the third month of human development, local epidermal thickenings form at the sites of future hairs: first on the eyebrows, chin, and upper lip and then over the rest of the thin skin. Cells at the base of each thickening invade the dermis, and a small dermal papilla invades the leading edge of the epidermal downgrowth. Interactions between the papilla and the invaginating epidermis induce the differentiation of the hair follicle. Hair begins to form in the hair bulb at the base of the hair follicle as a result of the keratinization of the bulb's epithelial cells. These cells are pushed toward the surface by the mitosis in the germinal matrix (hair bulb epithelium). Some epithelial cells in the walls of the developing follicle divide, forming bulges that differentiate into sebaceous glands. integumentary system
2. Hairs. By the fifth or sixth month of gestation, the fetus is covered by fine hairs (lanugo). Just before birth, most of the lanugo is shed, except for the scalp, eyebrows, and eyelashes. A few months after birth, the remaining lanugo has been replaced by coarser mature terminal hairs; the rest of the body is covered with a coat of fine short hairs, called vellus. At puberty, coarse terminal hairs replace the vellus in specific body areas. In males, terminal hairs develop in the axilla and pubic region, on the face, and, to some extent, over the rest of the body. In females, they develop mainly in the axilla and pubic regions.
nA. Follicle and Hair Development:
1. Follicles. Early in the third month of human development, local epidermal thickenings form at the sites of future hairs: first on the eyebrows, chin, and upper lip and then over the rest of the thin skin. Cells at the base of each thickening invade the dermis, and a small dermal papilla invades the leading edge of the epidermal downgrowth. Interactions between the papilla and the invaginating epidermis induce the differentiation of the hair follicle. Hair begins to form in the hair bulb at the base of the hair follicle as a result of the keratinization of the bulb's epithelial cells. These cells are pushed toward the surface by the mitosis in the germinal matrix (hair bulb epithelium). Some epithelial cells in the walls of the developing follicle divide, forming bulges that differentiate into sebaceous glands. integumentary system
2. Hairs. By the fifth or sixth month of gestation, the fetus is covered by fine hairs (lanugo). Just before birth, most of the lanugo is shed, except for the scalp, eyebrows, and eyelashes. A few months after birth, the remaining lanugo has been replaced by coarser mature terminal hairs; the rest of the body is covered with a coat of fine short hairs, called vellus. At puberty, coarse terminal hairs replace the vellus in specific body areas. In males, terminal hairs develop in the axilla and pubic region, on the face, and, to some extent, over the rest of the body. In females, they develop mainly in the axilla and pubic regions.
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